linux: Do not allow slab caches to be merged

linux: Do not allow slab caches to be merged

From the kernel documentation:

> For reduced kernel memory fragmentation, slab caches can be
> merged when they share the same size and other characteristics.
> This carries a risk of kernel heap overflows being able to
> overwrite objects from merged caches (and more easily control
> cache layout), which makes such heap attacks easier to exploit
> by attackers. By keeping caches unmerged, these kinds of exploits
> can usually only damage objects in the same cache. [...]

Thus, it is more sane to leave slab merging disabled. KSPP and ClipOS
recommend this as well.

Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
Acked-by: Michael Tremer <michael.tremer@ipfire.org>